Within the context of this specification, information is considered to be the meaning that a human assigns to data by means of the known conventions used in their representation. Data is considered to be a representation of facts, concepts, or instructions in a formalized manner suitable for communication, interpretation, or processing by humans or, in particular, by electronic device processors.
It is well known in the art that information to be transmitted is converted into data and is transmitted, using a frame format, in digital and or analogue form. A transmission frame is considered to be a repeated data structure, often beginning and ending with delimiters, such as prefixes, that consist of fields predetermined by a protocol for the transmission of payload data and control data. In the context of GPS, the term frame can be used to include reference to the terms word, sub-frame and frames, each of these having a repeated data structure and having a specific meaning within GPS.
A protocol is a formal set of conventions governing the format and control of interaction among communicating functional units, such as transmitters and receivers. The fields, and thus the transmission frame, are made up of bits, which represent the data in a binary digit form. The protocol governs the form of the repeated data structure.
Payload data is considered to be the data which is the subject of the transmission, and control data is the data which is required to transmit the user data. Control data may include, for example, the prefixes, frame length information (particularly in the case of variable frame length frames), and error correcting codes/parity bits.
Bit synchronisation is considered to refer to the identification of the boundary of bits i.e. allows the recognition of bits. Frame/word synchronisation occurs at a layer above bit synchronisation, and is considered to be the identification of the boundaries of frames i.e. allows the recognition of frames. These processes are required prior to actually interpreting the content of the frame.
Bit and frame synchronisation are key in transmission systems. Transmission systems are affected by noise. Solutions are required which are robust under noisy transmission condition. Some aspects and embodiments of the present invention address the problem of noise and their effect on frame synchronisation and their effect on the interpretation of frame content.
In order to meaningfully decode data contained in a sequence of frames (i.e. decipher the data content from the frames), the frame order of a frame in a sequence of frames must be obtained. This frame order data is provided within the frame structure. This deciphering process is done once bit and frame synchronisation have occurred. Aspects and embodiments of the present invention relate to the identification of the frame order of a frame in a sequence of frames.
Satellite systems utilise the time delay between a transmitted frame and the time at which the transmitted frame was received at a receiver. Basically, the time delay between the transmission and reception of the frame is used to work out the distance to the satellite, given the known transmission speed (speed of light) of the frame. Distances to a number of satellites are used to triangulate the position of a receiver.
Although four satellites are typically used for triangulation, a smaller number of satellites may be sufficient if external information is also available. This external information may be velocity, altitude from a barometer, or the propagation delay from a cellular basestation in a known location. Furthermore, if more than four satellites are visible, the signals from addition satellites can be used to improve accuracy.
Specifically, satellites provide the time at which the frame was transmitted, within the frame. The transmission times of each frame can be considered to be the frame order. This is because each frame will have its own transmission time and a subsequent frame will have a “later time” than an earlier frame.
The positioning satellite receiver utilises a local receiver time in the determination of time delay. Cellular assistance in GPS has made it possible to get local receiver time information from the cellular network. However, this is not a mandatory feature in GSM and WCDMA Standards. Furthermore, cellular assistance is also not available if the cellular network connection is lost. There are also technical and commercial problems in the availability of cellular assistance in GSM and WCDMA networks.
The existing GPS frame structure does not support time transfer (the time at which a signal is transmitted) when the satellite signal is weak. The time information is encoded in a 17 bit time-of-week field that is repeated once in 6 seconds. Thus, the time information field is prone to burst noise.
Specific embodiments of the present invention relate to satellite positioning signal design, especially the on-going design of Galileo signal frame structures. The present invention supports weak signal operation, particularly when accurate time assistance is not available from the cellular network, as is the case in most GSM and WCDMA networks.
As previously outlined, synchronisation of frames is essential in communication systems as it is necessary to know the boundaries of frames at the receiver side before decoding can take place. Commonly, but not always, the beginning of every fixed or variable length frame is identified by a contiguous sequence called a prefix. The prefix is used to identify the beginning of the frame. In the case of GPS, the prefix is used to synchronise the receiver.
Let us now turn to the issue of frame synchronisation in transmission systems.
To avoid the occurrence of the prefix elsewhere in the frame, one or more prefix synchronized codes (ps-code) are used for frame synchronisation in some transmission systems. However, they are not used in satellite navigation systems.
Ps-codes have the property that they modify the payload data such that the bit sequence corresponding to the the prefix does not occur in any codeword or in any concatenation of codewords in any position other than the first position i.e. the prefix is a special bit sequence that does not appear elsewhere in the bit stream, neither inside frames nor across the boundaries between the frames. Therefore, they can uniquely identify the beginning of a frame.
Prefix-synchronized codes have been proposed to facilitate frame/word synchronization in noiseless and noisy data transmission channels. Prefix-synchronized codes are discussed e.g. in E. N. Gilbert, “Synchronization of binary messages”, IRE Transactions of information theory, September 1960. A more recent discussion can be found in: A. J. Wijngaarden, “Partial-prefix synchronizable codes”, IEEE Transactions on information theory, no. 5, July 2001.
GPS signals already have certain repetitive bit patterns which act as prefixes. The patterns were not planned for weak signal synchronization and their usefulness for that purpose is marginal since they are not uniquely distinguishable, not repeated at constant time intervals, and appear infrequently. In U.S. Pat. No. 5,798,732, a method is described for using certain repetitive bit fields in GPS messages for timing.
Current transmissions systems, including existing GPS systems, assume that the prefixes are received error free. This assumption is not always valid. Aspects and embodiments of the present invention provide the analysis of prefixes to detect the timing of a repeated data structure, such as a word, or a sub-frame, or a frame. This can be in satellite transmissions systems.
Furthermore, aspects and embodiments of the present invention can be employed when the signal is so weak that data bits cannot be detected reliably enough for data reception. In such weak signal situations, the timing cannot be detected reliably in the prior art from ordinary frame delimiters or other similar bit patterns.